1. Field of the Invention
The present invention relates generally to ultrasonic transducers and their fabrication.
2. Related Art
Composite piezoelectric transducers are devices that comprise part sintered, densified ceramic (for example, lead zirconate titanate (PZT)) embedded in a polymer. The composite structure enhances the efficiency of electrical to mechanical energy conversion and propagation, and in reverse, mechanical to electrical energy conversion. The geometry of the structure can have many forms. One example geometry is a 1–3 composite. This 1–3 refers to a material where the electrical field is applied through the thickness of a plate and the mechanical motion is also in this direction.
The composite structure derives performance improvements in two main processes:
a) Firstly, the overall density of the composite is less that pure ceramic which in the case of PZT is approximately 8. This density and mechanical modulus gives rise, to an acoustic index of ˜30 megarayls (Mrayls). Most uses for transducers are either in medical or sonar applications where the medium of tissue or water has an acoustic index of about 1.5 Mrayls. This mismatch causes large coupling losses at the interface of the transducer and the material. A mechanical transformer in the form of matching layers of ¼ wavelength thickness of material with intermediate acoustic densities then needs to be inserted between the transducer and the material. The efficiency of energy transfer is thus limited to the affectivity of this mechanical transformer, which is compromised by the lack of materials available with acoustic indexes of about 6–20 Mrayls. By using a composite made up of a mixture of polymer and ceramic, a composite transducer can have an acoustic index of about 4–10 Mrayls depending on the percentage of ceramic in the transducer. This is much easier to couple to the target mediums and higher coupling efficiencies are achieved.
b) Secondly, under the stress of an electrical field a piezoelectric material (“piezo material”) will change its shape. This will cause internal stresses which hold the material in its solid form. If the piezo material is in the form of long, thin pillars, then the forces opposing the lengthening of the pillar will be the force of thinning the pillar that allows material to become part of the length extension. In a solid mass of ceramic this sideways resistance is substantial and only small displacements are possible. If the pillar is held in a polymer, then this resistance to thinning is minimal allowing the pillar to extend its length at a greater efficiency.
Composite material is made by a technique known as dice and fill where a solid block of sintered PZT is sliced into plates and then the plates are diced (partially cut over their flat surfaces with a thin blade) these cuts are then filled with a polymer and then the supporting back of the PZT is ground off leaving just pillars embedded in the polymer. This is an expensive and laborious task with quality and yield difficulties.